Refs #1845, switch to async JS in other places and apply fixes to tracking code admin page & other pages.

Notes:
  * Fix bug in Installation page that displays JS where newlines were not displayed and text would not be selected on click.
  * Modify piwik.js to make sure setTrackerUrl is called before _paq is processed.
  * Move _paq modifications in tracker code to global scope outside every anonymous function.
  * Fix bug in rewritten site selector widget where selecting the same site didn't fire an event (necessary for some uses).
  * Fix bug where change of site resulted in delay before site name in labels changed.
  * Fix bug where site name was double encoded in tracking code admin page.
  * Rewrite piwik_tag.tpl to use async tracking code.